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Summary of Northern Leaf Blight

Northern leaf blight (NLB) is found in humid climates wherever corn is grown. It has spread in recent years due to major weather events, especially hurricanes, which carry the organism from south to north in North America.

Northern leaf blight is caused by the fungus Exserohilum turcicum. Multiple “races” have been identified in the U.S.

Yield losses are most severe when northern leaf blight infects corn plants early and then progresses to the upper leaves by early ear fill. Slowing disease progression relative to crop development reduces the impact of the disease.

Genetic resistance to the northern leaf blight races is available in corn. Due to race shifts and multiple races in some areas, DuPont Pioneer corn breeders are incorporating multiple resistance genes into hybrids.

DuPont Pioneer rigorously evaluates and characterizes hybrids for resistance to Northern Leaf Blight so that growers have critical information to use in hybrid selection.

Disease Development and Symptoms

Northern corn leaf blight is caused by the fungus Exserohilum turcicum, also known as Setosphaeria turcica, and previously known as Helminthosporium turcicum (Figure 1). The disease organism overwinters as mycelia and conidia in diseased corn leaves, husks and other plant parts (Figure 2). Spores are produced on this crop residue when environmental conditions become favorable in spring and early summer. These spores are spread by rain splash and air currents to the leaves of new crop plants, where primary infections are produced. Infection occurs when free water is present on the leaf surface for 6 to 18 hours and temperatures are 65 to 80 F.

Figure 1. NLB symptoms on leaf of susceptible corn hybrid.

Secondary spread occurs from plant to plant and field to field as spores are carried long distances by the wind. Infections generally begin on lower leaves and then progress up the plant However, in severe NLB outbreak years (that have high spore levels), infections may begin in the upper plant canopy. This can occur when weather systems deposit spores from southern growing areas such as Mexico and the Caribbean. In recent years, weather patterns with large storms moving from south to north over the North American continent have spread the NLB organism into additional northern regions.

Heavy dews, frequent light showers, high humidity and moderate temperatures favor the spread of NLB. Development of disease lesions on the ear leaf or above and significant loss of green leaf area can result in yield loss.

Figure 2. NLB Disease Cycle.

Races of Northern Leaf Blight

There are multiple races of Setosphaeria turcica documented in North America; Race 0, Race 1 and Race 23N are the most prevalent. Ferguson and Carson (2007) reported a survey of NLB races which indicated that frequency of Race 0 isolates decreased from 83% in 1974 to 50% in the 1990s. During this same period, Race 1 isolate frequency increased. Low levels of Race 23 and 23N were present throughout the 20-plus years. The authors attribute the decrease in Race 0 frequency to the widespread use of the Ht1 gene by the sweet corn and hybrid corn industries, which has provided control of Race 0 but not of Race 1.

The resistance genes available to corn breeders are named "Ht" based on the previous NLB fungal name (H)elminthosporium (t)urcicum. The common sources of resistant Ht genes are dominant genes and provide resistance to the various key races of Exserohilum turcicum (Et) as shown in Table 1.

Table 1. Common sources of resistance Ht genes.

DuPont Pioneer Breeders Target Multiple NLB RacesTo provide disease resistance to NLB when multiple races might be present, 2 or more Ht genes may be needed. For example, a combination of Ht1 and Ht2 genes would provide resistance to Races 0, 1 and 23N, the predominant races of NLB in the U.S. and Canada. Because of these multiple races of NLB, DuPont Pioneer breeders are incorporating additional Ht genes in their hybrid development programs (i.e., a "multigenic" approach). Resistant phenotype and inheritance of NLB resistance genes are shown below (Table 2):

Table 2. "Ht" resistance genes. (Welz and Geirger, 2000)

1 sht1 is a dominant inhibitor of Ht2, Ht3, and Htn1 (but not of Ht1) in some parent lines.

The resistant phenotype which appears with Ht1, Ht2 and Ht3 genes is tissue chlorosis, where normal green color begins to change to a yellow hue in leaf lesions (Figure 3). These northern leaf blight lesions are slower to develop, and there are fewer spores produced per lesion.

With the Ht4 gene, a chlorotic "halo" appears around the lesions, which are somewhat smaller in size and fewer in frequency.

The Htn1 gene prolongs the latent period before lesions occur; fewer and smaller northern leaf blight lesions develop with fewer spores produced per lesion. The plant is able to maintain its health longer even with the disease organism present (Figure 3). The Htm1 and NN genes provide complete resistance, and minimal lesions are noted in plants with these genes present.

Evaluation and Characterization of Corn Hybrids for NLB Reaction

DuPont Pioneer evaluates corn hybrids in multiple environments to observe their reaction to NLB infection. Inoculated plots as well as "natural infection" sites are used to establish disease pressure. Both basic research trials (small plots) and advanced testing trials (larger IMPACT™ plots) are used for this hybrid characterization process. Use of numerous widespread locations, including those with a history of extreme NLB incidence, helps ensure that some environments will provide severe NLB pressure to challenge even the best hybrids. It also helps provide exposure of hybrids to as many race variants of NLB as possible. The critical time for evaluating disease damage begins in the early reproductive stages of development.

The DuPont Pioneer 1 to 9 NLB scoring system is based on "leaf loss" from the disease; a score of "9" indicates no leaf loss and a score of "1" denotes 95% leaf loss in the presence of the disease (Figure 7). In determining overall hybrid ratings, experimental hybrids are compared to hybrids of "known" response to NLB. This provides a "relative" rating system in which new hybrids are characterized as accurately as possible relative to established hybrids that are more familiar in the marketplace.

Managing Northern Leaf Blight in Corn Production

Resistant HybridsSelection of resistant hybrids based on disease reaction characterization scores is an important first step in managing this disease. The Pioneer NLB rating reflects the hybrids' expected performance against the major NLB races predominant in your area. As race shifts inevitably occur, continued testing by DuPont Pioneer researchers may result in a rating adjustment for some hybrids. Use of multigenic resistance by breeders increases hybrid stability as NLB races shift over time.

Hybrids should be selected based on all important traits needed for a field. In addition to NLB resistance, select hybrids with high yield potential, appropriate insect resistance traits, suitable (usually full-season) maturity for the area, and data from multiple locations and years that demonstrate consistent performance. Strong emergence, stalk strength, and drought tolerance are other agronomic characteristics to consider to help optimize stands and harvestable grain yields.

Reducing Previous Corn ResidueReducing corn residue decreases the amount of NLB inoculum available to infect the subsequent crop. Crop rotation is 1 effective method of reducing residue. In addition, any form of tillage that places soil in contact with corn residue promotes decomposition and decreases the amount of residue that survives to the subsequent cropping season. Stover harvest for cellulosic ethanol production or animal feed is another means to reduce corn residue and disease inoculum. However, reducing corn residue does not protect against spore showers carried into a field on wind currents.

Timely PlantingTimely planting can often help hybrids escape the most severe damage from NLB if crop development outpaces normal disease progression. The latest-planted corn in an area may be infected when plants are smaller, resulting in the disease progressing more rapidly relative to the crop. However, in cases of high disease incidence, both early- and late-planted corn may be severely damaged.

Fungicide ApplicationVarious foliar fungicides are available to help control or suppress NLB development (Table 3). Though fungicides are routinely used by growers to protect against several common leaf diseases, NLB may not always be controlled as completely as some other diseases. This is because of the more rapid life cycle of NLB, which may be as short as 1 week under favorable conditions. Because NLB sporulates so rapidly, it is more difficult to time a single fungicide application. Consequently, selecting resistant hybrids is a crucial first step in managing NLB where incidence is historically high.

Decisions to use a fungicide must be based on the disease risk factors of the field, including hybrid susceptibility, cropping sequence, tillage system, location, disease history, yield potential, the price of corn, and expected weather during reproductive development. In fact, weather conditions anticipated during ear fill are a primary factor for disease development and often have the most impact (along with hybrid disease rating) on the profitability of fungicide applications.

Survey results from 374 on-farm trials where previous crop and tillage practices were reported showed an inverse relationship between tillage intensity and yield response to foliar fungicide application in both corn following corn and corn following soybean (Figure 8). These results indicate that rotation and tillage have a positive impact on reducing disease pressure.

Other studies (results not shown) show a similar relationship between hybrid disease rating and yield response to fungicides; the more resistant the hybrid, the less advantage achieved by fungicide application. Hybrids with a score of "6" or greater often show little or no economic benefit from a fungicide application under moderate infestation levels. Fungicides for NLB management are shown in Table 3.

c Fungicide performance is variable and subject to a variety of environmental and disease pressures. Individual results may vary.

d Always read and follow all label directions and precautions for use when applying fungicides. Labels contain important precautions, directions for use and product warranty and liability limitations that must be read before using the product.